This invention relates generally to processes for recovering citric acid, and more particularly to such processes involving adsorption and desorption of citric acid to and from a solid-phase polymer.
By way of further background, citric acid is a biologically occurring material which finds primary use in the food industry. It has gained worldwide acceptance as a food ingredient, and, has a pleasant acid taste and high water solubility which have motivated its most extensive food application in beverages, jams, jellies, and sweets. Additional uses of citric acid are also prevalent. For instance, it is used in the pharmaceutical and cosmetics industries, in the plastics industry as a raw material for the manufacture of citric acid ester plasticizers, as well as more recently finding substantial use in the preparation of detergents and other cleaning agents.
Since the early work of C. Wehmer beginning in the 1890's, there has been substantial interest and investment in fermentation processes for producing citric acid. As such, nearly all of the 700 million pounds or more of citric acid produced worldwide yearly are from fermentation processes, for instance by the fermentation of a carbon source such as molasses with the microorganism, Aspergillus niger. Typically, broths from such fermentations will contain about 10 weight % or more citric acid, as well as about 1000 ppm or more salts, about 1 weight % carbohydrates, and 2 weight % proteins, amino acids and other materials.
As will be appreciated, the recovery of the citric acid product from such mediums has itself been the subject of substantial attention in the academia and industry. In general, three techniques have been used to date, those being precipitation, solvent extraction and solid-phase polymer adsorption and subsequent desorption. As to the first technique, precipitation, it can be fairly stated that it has been the more preferred technique used on a commercial scale, even though significant endeavors as to the other two techniques have been made. In precipitation, calcium hydroxide (lime) is usually added to the fermented medium to form the slightly soluble tricalcium citrate tetrahydrate. Properly performed, this precipitation leaves most impurities in the solution. Impurities may further be removed by washing the filtered precipitate. To further purify the product, the moist precipitate is reacted with sulfuric acid to yield calcium sulfate (gypsum) and a solution of free acid. The free acid solution is then treated with activated carbon and ion exchange resins before evaporation to the crystalline citric acid product. As is recognized, the efficacy of this precipitation method is highly dependent on properly and carefully performing the various steps involved. It is thus a sensitive process requiring high refinement, especially on a commercial scale.
A second technique which has been used to recover citric acid is solvent extraction. In this technique, citric acid is extracted from the fermentation broth with solvent hydrocarbons, for example, octane, benzene, kerosene, ethers, esters, ketones or amines. Citric acid is then reextracted from the solvent phase into water with either the addition of heat or the formation of a citric acid salt. However, this solvent extraction technique is also expensive and complex. Further, solvent extraction generates a very substantial amount of waste for disposal, which from both cost and environmental standpoints is unattractive.
A third technique which has been suggested but to applicants' knowledge not applied on a commercial scale involves the use of solid adsorbents to remove citric acid from the medium. The adsorbed citric acid is then recovered from the polymer utilizing a desorbing agent. For example, U.S. Pat. No. 4,323,702 to Kawabata et al. describes a process for recovering carboxylic acids with a material of which the main component is a polymeric compound having a pyridine skeletal structure and a cross-linked structure. As the patent directs, the captured carboxylic acids are then desorbed using an aliphatic alcohol, an aliphatic ketone or a carboxylic ester as the desorbing agent.
U.S. Pat. No. 4,720,579 to Kulprathipanja describes a process in which citric acid is separated from a fermentation broth using an adsorbent of a neutral, noniogenic, macroreticular, water-insoluble cross-linked styrene-poly(vinyl)benzene. In this patent, a number of pulse tests were run using the described adsorbents. In these tests, a helical column was filled with the adsorbent, and a liquid described as the desorbent passed through the column. At a convenient time, a pulse of feed containing known concentrations of citric acid and other components was injected into the column. The manner in which the injected materials came off the column was then studied. For example, several tests were run in which water was passed through the column, although the patent cautioned that using water, increased desorption temperatures so as to cause premature deactivation of the adsorbent were required. Thus, the patent described a solution to this problem which included adding acetone in about 1 to 15% to the water as desorbent.
In U.S. Pat. No. 4,851,573 to Kulprathipanja et al., another process is described in which an adsorbent of a cross-linked acrylic or styrene resin matrix having attached tertiary amine functional groups or pyridine functional groups is used as an adsorbent. Exemplified in the patent are again pulse tests similar to those described in the above-identified Kulprathipanja '579 patent, this time at temperatures of 60.degree. to 75.degree. C. The adsorbents exemplified included an acrylic resin having attached modified tertiary amines groups functionalized with sulfate ions, and polystyrene resins having attached pyridine groups functionalized with sulfate ions. The patent exemplified addition of sulfuric acid to citric acid solutions prior to the adsorption step, which helps drive functionalization to completion. The Kulprathipanja '573 patent further identifies sulfuric acid and other inorganic acids and water as desorbents, but directs a strong preference for the dilute sulfuric acid because others will be "found to be less effective" and specifically in some cases states that water "is not strong enough to recover the absorbed citric acid quickly enough to make the process commercially attractive."
In still another patent, U.S. Pat. No. 4,851,574, Kulprathipanja describes separating citric acid from a fermentation broth using an adsorbent of a cross-linked acrylic or styrene resin matrix having attached aliphatic quaternary amine functional groups. Again, pulse tests are exemplified, in which sulfuric acid is demonstrated as a desorbent. These pulse tests were conducted at temperatures ranging from 50.degree. to 60.degree. C.
In light of the above and other literature, there remains a need for a highly commerically attractive process for recovering citric acid from a fermentation broth or other medium, by which the citric acid is recovered in a form that is readily purifiable and free from unnecessary organic or inorganic impurities which need be removed. The above-described work by Kawabata utilized organic solvents as desorbing agents which need be removed, thus adding expense and complication to the purification process. Moreover, where alcohol is used as desorbent, upon concentration of the desorbed medium or evaporation of solvent, significant esterification occurs thus leading to additional undesirable impurities in the product. The above-described work by Kulprathipanja et al. harbors instability due to the resins used, and clearly directs that preferred desorbents include either inorganic acids or organic solvent materials which are processed through the column along with the citric acid product and thereby substantially contaminate the same and necessitate significant additional purification measures. Advantageous processes are therefore needed utilizing resins which minimize possible impurities in the product, as well as avoid the need to purify substantial amounts of inorganic acid desorbents such as sulfuric acid or organic desorbents such as aliphatic alcohols, ketones and carboxylic esters from the product. The applicants' invention addresses these needs and provides for the first time commercially attractive processes employing highly stable adsorbents as well as desorbent steps which minimize the opportunity for unnecessary impurities in the desorbed product and greatly simplify workup procedures.